The prostate is one of the male reproductive organs found in the pelvis below the urinary bladder. It functions to produce and store seminal fluid which provides nutrients and fluids that are vital for the survival of sperm introduced into the vagina during reproduction. Like many other tissues, the prostate glands are also prone to develop either malignant (cancerous) or benign (non-cancerous) tumors. The American Cancer Society predicted that over 230,000 men would be diagnosed with prostrate cancer and over 30,000 men would die from the disease in year 2005. In fact, prostate cancer is one of the most common male cancers in western societies, and is the second leading form of malignancy among American men. Current treatment methods for prostrate cancer include hormonal therapy, radiation therapy, surgery, chemotherapy, photodynamic therapy, and combination therapy. The selection of a treatment generally varies depending on the stage of the cancer. However, many of these treatments affect the quality of life of the patient, especially those men who are diagnosed with prostrate cancer over age 50. For example, the use of hormonal drugs is often accompanied by side effects such as osteoporosis and liver damage. Such side effects might be mitigated by the use of treatments that are more selective or specific to the tissue being responsible for the disease state, and avoid non-target tissues like the bones or the liver. As described herein, prostate specific membrane antigen (PSMA) represents a target for such selective or specific treatments.
PSMA is named largely due to its higher level of expression on prostate cancer cells; however, its particular function on prostate cancer cells remains unresolved. PSMA is over-expressed in the malignant prostate tissues when compared to other organs in the human body such as kidney, proximal small intestine, and salivary glands. Though PSMA is expressed in brain, that expression is minimal, and most ligands of PSMA are polar and are not capable of penetrating the blood brain barrier. PSMA is a type II cell surface membrane-bound glycoprotein with ˜110 kD molecular weight, including an intracellular segment (amino acids 1-18), a transmembrane domain (amino acids 19-43), and an extensive extracellular domain (amino acids 44-750). While the functions of the intracellular segment and the transmembrane domains are currently believed to be insignificant, the extracellular domain is involved in several distinct activities. PSMA plays a role in central nervous system, where it metabolizes N-acetyl-aspartyl glutamate (NAAG) into glutamic and N-acetyl aspartic acid. Accordingly, it is also sometimes referred to as an N-acetyl alpha linked acidic dipeptidase (NAALADase). PSMA is also sometimes referred to as a folate hydrolase I (FOLH I) or glutamate carboxypeptidase (GCP II) due to its role in the proximal small intestine where it removes γ-linked glutamate from poly-γ-glutamated folate and α-linked glutamate from peptides and small molecules.
PSMA also shares similarities with human transferrin receptor (TfR), because both PSMA and TfR are type II glycoproteins. More specifically, PSMA shows 54% and 60% homology to TfR1 and TfR2, respectively. However, though TfR exists only in dimeric form due to the formation of inter-strand sulfhydryl linkages, PSMA can exist in either dimeric or monomeric form.
Unlike many other membrane-bound proteins, PSMA undergoes rapid internalization into the cell in a similar fashion to cell surface bound receptors like vitamin receptors. PSMA is internalized through clathrin-coated pits and subsequently can either recycle to the cell surface or go to lysosomes. It has been suggested that the dimer and monomer form of PSMA are inter-convertible, though direct evidence of the interconversion is being debated. Even so, only the dimer of PSMA possesses enzymatic activity, and the monomer does not.
Though the activity of the PSMA on the cell surface of the prostate cells remains under investigation, it has been recognized by the inventors herein that PSMA represents a viable target for the selective and/or specific delivery of biologically active agents, including diagnostic agents, imaging agents, and therapeutic agents to such prostate cells.
It has been discovered that biologically active compounds that are conjugated to ligands capable of binding to prostate specific membrane antigen (PSMA) via a linker may be useful in the imaging, diagnosis, and/or treatment of prostate cancer, and related diseases that involve pathogenic cell populations expressing or over-expressing PSMA. PSMA is a cell surface protein that is internalized in a process analogous to endocytosis observed with cell surface receptors, such as vitamin receptors. Accordingly, it has been discovered that certain conjugates that include a linker having a predetermined length, and/or a predetermined diameter, and/or preselected functional groups along its length may be used to treat, image, and/or diagnose such diseases.
In one illustrative embodiment of the invention, conjugates having the formulaB-L-Dare described wherein B is a prostate specific membrane antigen (PSMA) binding or targeting ligand, L is a linker, and D is a drug. As used herein, the term drug D collectively includes therapeutic agents, cytotoxic agents, imaging agents, diagnostic agents, and the like, unless otherwise indicated or by the context. For example, in one illustrative configuration, the conjugate described herein is used to eliminate a pathogenic population of cells and therefore the drug D is a therapeutic agent, a cytotoxic agent, and the like. In another illustrative configuration, the conjugate described herein is used to image and/or diagnose a disease or disease state, and therefore the drug D is an imaging agent, a diagnostic agent, and the like. Other configurations are also contemplated and described herein. It is to be understood that analogs and derivatives of each of the foregoing B, L, and D are also contemplated and described herein, and that when used herein, the terms B, L, and D collectively refer to such analogs and derivatives.
In one illustrative embodiment, the linker L may be a releasable or non-releasable linker. In one aspect, the linker L is at least about 7 atoms in length. In one variation, the linker L is at least about 10 atoms in length. In one variation, the linker L is at least about 14 atoms in length. In another variation, the linker L is between about 7 and about 31, between about 7 and about 24, or between about 7 and about 20 atoms in length. In another variation, the linker L is between about 14 and about 31, between about 14 and about 24, or between about 14 and about 20 atoms in length.
In an alternative aspect, the linker L is at least about 10 angstroms (Å) in length. In one variation, the linker L is at least about 15 Å in length. In another variation, the linker L is at least about 20 Å in length. In another variation, the linker L is in the range from about 10 Å to about 30 Å in length.
In an alternative aspect, at least a portion of the length of the linker L is about 5 Å in diameter or less at the end connected to the binding ligand B. In one variation, at least a portion of the length of the linker L is about 4 Å or less, or about 3 Å or less in diameter at the end connected to the binding ligand B. It is appreciated that the illustrative embodiments that include a diameter requirement of about 5 Å or less, about 4 Å or less, or about 3 Å or less may include that requirement for a predetermined length of the linker, thereby defining a cylindrical-like portion of the linker. Illustratively, in another variation, the linker includes a cylindrical portion at the end connected to the binding ligand that is at least about 7 Å in length and about 5 Å or less, about 4 Å or less, or about 3 Å or less in diameter.
In another embodiment, the linker L includes one or more hydrophilic linkers capable of interacting with one or more residues of PSMA, including amino acids that have hydrophilic side chains, such as Ser, Thr, Cys, Arg, Orn, Lys, Asp, Glu, Gln, and like residues. In another embodiment, the linker L includes one or more hydrophobic linkers capable of interacting with one or more residues of PSMA, including amino acids that have hydrophobic side chains, such as Val, Leu, Ile, Phe, Tyr, Met, and like residues. It is to be understood that the foregoing embodiments and aspects may be included in the linker L either alone or in combination with each other. For example, linkers L that are at least about 7 atoms in length and about 5 Å, about 4 Å or less, or about 3 Å or less in diameter or less are contemplated and described herein, and also include one or more hydrophilic linkers capable of interacting with one or more residues of PSMA, including Val, Leu, Ile, Phe, Tyr, Met, and like residues are contemplated and described herein.
In another embodiment, one end of the linker is not branched and comprises a chain of carbon, oxygen, nitrogen, and sulfur atoms. In one embodiment, the linear chain of carbon, oxygen, nitrogen, and sulfur atoms is at least 5 atoms in length. In one variation, the linear chain is at least 7 atoms, or at least 10 atoms in length. In another embodiment, the chain of carbon, oxygen, nitrogen, and sulfur atoms are not substituted. In one variation, a portion of the chain of carbon, oxygen, nitrogen, and sulfur atoms is cyclized with a divalent fragment. For example, a linker (L) comprising the dipeptide Phe-Phe may include a piperazin-1,4-diyl structure by cyclizing two nitrogens with an ethylene fragment, or substituted variation thereof.
In another embodiment, pharmaceutical compositions are described herein, where the pharmaceutical composition includes the conjugates described herein in amounts effective to treat diseases and disease states, diagnose diseases or disease states, and/or image tissues and/or cells that are associated with pathogenic populations of cells expressing or over expressing PSMA. Illustratively, the pharmaceutical compositions also include one or more carriers, diluents, and/or excipients.
In another embodiment, methods for treating diseases and disease states, diagnosing diseases or disease states, and/or imaging tissues and/or cells that are associated with pathogenic populations of cells expressing or over expressing PSMA are described herein. Such methods include the step of administering the conjugates described herein, and/or pharmaceutical compositions containing the conjugates described herein, in amounts effective to treat diseases and disease states, diagnose diseases or disease states, and/or image tissues and/or cells that are associated with pathogenic populations of cells expressing or over expressing PSMA.